The present invention relates to a headbox of a paper machine or board machine and more particularly to the turbulence generator in the headbox, following the transverse distributor.
A headbox in which the invention might be used is disclosed in German Patent Application DE 44 37 180 of the applicant. This headbox has a machine width transverse distributor, a turbulence generating region following downstream and supplied by the distributor and a headbox nozzle following the turbulence generating region downstream. The turbulence generating region contains a grating, a following equalization chamber and a tube bundle. The tube bundle widens stepwise in the flow direction of the stock suspension, and the tube bundle has the greatest tube diameter at the outlet end of the tube bundle. The tubes of the tube bundle turbulence generator thus have considerably smaller inlet cross sections than outlet cross sections. One reason for this is that a minimum land area must be provided on the inflow side to prevent the formation of fiber clumps and contamination. Abrupt cross-sectional widenings are provided in the tube to generate specifically desired turbulences for ensuring that the fiber flocs in the suspension are broken up. This has a positive influence on the later sheet formation.
To avoid disturbing wake effects in the following downstream nozzle, it is necessary to keep the land areas at the outlet of the tube bundle small. To satisfy the requirement for small land areas at the outlet end of the turbulence generating region, the outlet cross sections of the tube bundles are usually not circular in any headbox, but rather have a shape which permits their highest possible packing density. Because the tubes are not of circular shape in the end region of the turbulence generator, secondary flows form in the tubes, and these flows lead to disturbances which can penetrate as far as the nozzle outlet gap of the following downstream nozzle. This penetration of disturbances ultimately leads to a negative influence on the formation of the sheet, and hence to impairment of the final paper quality.
The inventors have discovered that known turbulence generating concepts cause the following typical disturbances:
1. As a consequence of secondary flows in the divergent outlet region of the tubes, transverse flows are produced, and these cannot be dissipated completely in the following nozzle. These transverse flows are reinforced by the flow deflection upstream of the slice at the nozzle outlet, and they are visible in the jet as regular furrows. A disturbed jet leads to a streaky formation of the sheet.
2. A streaky formation may likewise be produced as a consequence of demixing in the tube corners.
3. If baffles are connected downstream of the turbulence tubes, the baffles have to extend over a significant part of the flow path in order to be able to reduce the above described turbulence tube disturbances. Microturbulence, which can partly eliminate the described disturbances, is produced at the baffle surface as a result of friction between the fluid and the tube wall.
Complete elimination of the disturbances is not possible, because of the short wavelength of the microturbulence and the comparatively low energy content of these turbulent transverse movements. Although the disturbances described are further dissipated with increasing baffle length, it is disadvantageous that, because of the then increasing microturbulence, an undesirably hard, fine grained formation of the paper web is likewise produced.
In practice, the selection of the baffle length thus always constitutes a compromise between adequate elimination of disturbances, on the one hand, and the least possible negative influence on the sheet formation, on the other hand. Adequate elimination of the disturbances, which are caused by the turbulence generators which are common currently, is not possible by using baffles connected downstream. All the headboxes built nowadays therefore produce streaky disturbances of the formation under critical operating conditions.
4. In perforated roll headboxes, with perforated rolls as turbulence generators, it is necessary, for static strength reasons, for the land area of the perforated roll to be greater than about 55%. The large webs which are produced thereby cause coarse turbulence during the passage of the flow, and this turbulence often cannot decay adequately in the headbox nozzle and, as a consequence, also causes disturbances to the formation.
For single layer and multilayer headboxes, it has been shown that the disturbances in headboxes with tube bundle turbulence generators, the disturbances generated by the abrupt steps and/or by widenings of the turbulence tubes, and the disturbances which are brought about at the outlet from the perforated roll in headboxes having turbulence generators constructed as a perforated roll all cannot be reduced to an adequate extent with the currently conventional geometries of the elements which follow in the headbox. Even slight convergent widenings in one plane can lead to transverse flows which cannot be eliminated, particularly at the limits of the operating range of the headbox. This means, therefore, that the turbulence generating unit upstream of the nozzle must be dimensioned such that far fewer disturbances, in the form of stationary irregularities, are caused by this turbulence generating element.
Since the influences of disturbances which are produced by the turbulence generator are also determined to a significant extent by the dimensions of the turbulence generating passages, it is expedient to relate the configuration of a headbox to a significant extent to the hydraulic diameter of the turbulence generating region. The hydraulic diameter d.sub.hydr is defined as four times the total cross-sectional area through which fluid flows, divided by the length of all the edge regions which occur. In an ideal, circular cross section, this corresponds exactly to the geometric diameter of the circular area. In an infinitely long gap, the hydraulic diameter is twice the height of the gap.